1. Field of the Invention
The present invention relates to lasers and to apparatus for alignment-stable, reflectance-outcoupled electro-optically Q-switched laser resonators.
2. Background Information
The utility of lasers derives from the unique properties of laser light, including high brightness, monochromaticity, low beam divergence, and coherence. These attributes make the radiation from lasers completely different than the radiation from previously known sources, and open up a wide area of application that includes rangefinding, tracking, motion sensing, communications, seismography, holography, and various defense applications.
Many communications and measurement applications of laser technology require series of pulses of laser radiation which have prescribed energy levels, periods of pulse duration, and intervals between pulses. A series of laser pulses may be produced by a laser which operates in conjunction with a Q-switch. A Q-switch is an apparatus which is used to obtain short, intense bursts of oscillation from lasers. These devices are well known in the electro-optics arts and are described in the text Optoelectronics: An Introduction, by J. Wilson and J. F. B. Hawkes, published by Prentice-Hall in 1983.
Q-switching techniques may utilize active or passive components to introduce time or intensity dependent losses into the laser resonator. A Q-switch constrains lasing action within a laser medium by preventing the buildup of oscillations within the resonator. The switching which occurs is a change in the Q or quality factor of the resonator. The quality factor is a quotient which is proportional to the energy stored in a resonator divided by the energy dissipated per cycle. A Q-switch functions by altering the optical pathway within a resonator. When a high loss condition is imposed on a laser cavity, energy which enters the laser medium from an excitation source is dissipated before atoms or molecules in the medium can be stimulated to laser oscillation. If a large population inversion is built up within the laser medium and the Q of the resonator is suddenly increased by eliminating the cavity loss conditions, laser action will suddenly begin. Once the quality factor of the resonator is sharply increased, laser oscillations build-up rapidly within the laser cavity and all available energy is emitted in one, large pulse which substantially depopulates the upper lasing level and shuts down the laser oscillation.
Q-switching may be accomplished using an active component such as a rotating mirror or prism. This technique requires one of the resonator reflectors to be rotated at a high angular velocity so that optical losses within the resonator prevent lasing except for the brief interval in each rotation cycle when the reflectors are substantially parallel. Such systems utilize mirrors or prisms that rotate up to 60,000 revolutions per minute and require costly motor and control apparatus which are susceptible to mechanical failure and misalignment.
Electro-optic, magneto-optic, and acousto-optic modulators may also be used as Q-switches. An electro-optic crystal, for example, may be placed within a laser resonator and alter the quality factor of the cavity by changing the polarization of the radiation in the cavity in response to an electrical signal applied to the crystal. Acoustical devices are employed to deflect some of the laser beam out of the cavity by diffracting light off sound waves and thereby diminish the quality factor of the resonator.
Saturable absorbers can be employed as passive Q-switches. A transparent cell containing a normally opaque, bleachable dye in a suitable solvent is placed within a laser cavity in order to absorb some portion of incident excitation energy. When the dye saturates and can no longer absorb excitation radiation, the dye loses its original opacity and functions as a Q-switch by increasing the optical efficiency of the laser.
The optical feedback loop of a laser resonator fundamentally comprises an active gain medium bounded by reflectors at each end of the resonator cavity. These reflectors must be maintained in precise alignment so that laser radiation emanating from the gain medium can make a myriad of transits through the medium between the reflectors. Resonators are susceptible to high energy losses if the reflectors are even slightly misaligned. Losses large enough to completely disable a resonator can be generated if one of the resonator reflectors is tilted off its alignment axis by only a few milliradians. The use of a Q-switch within a resonator cavity exacerbates typical systemic misalignment losses.
Another source of energy losses which plagues the operation of a laser resonator is dimensional instability within the resonator cavity. Dimensional instability results from the unwanted relative movement of components of the laser resonator. Any change in the optical alignment of the resonator components produces a loss which adversely affects the laser output power and beam quality.
One of the major concerns in designing and fabricating a laser resonator is to substantially eliminate dimensional instability by increasing the alignment insensitivity of the resonator.
For laser systems that are manufactured in large quantities, resonating cavity designs that employ a minimum number of inexpensive part are highly desirable. Moreover, the design should ideally result in a laser cavity and means of radiation outcoupling that result in a performance which is not degraded by temperature change, physical shock, and other environmental hazards; particularly if the laser system is used in the field.
The reflectors placed at either end of the cavity need not be plane mirrors, but must be aligned so that multiple reflections occur with little loss. The resonator cavity is usually very sensitive to changes in alignment of the mirrors. A small tilt of one mirror, for example, subjects the resonator cavity to a large loss of energy. Typically, a relatively small misalignment in the resonator cavity can prevent the operation of the laser transmitter. In addition, the loss of energy through misalignment is made more likely by the use of Q-switching. Q-switches retard stimulated emission by preventing the buildup of oscillations in the resonant cavity. When a high level of population inversion is reached in the laser medium, a Q-switch is triggered to suddenly restore the resonant cavity.
Previous efforts to achieve alignment insensitivity have included the use of concave mirrors, opposite corner-cube reflectors tilted against correlative Brewster angles, and internal reflection prisms arranged with either parallel or obliquely crossed rooflines. In U.S. Pat. No. 4,420,836, Harper discloses an optical resonator cavity configuration using a unitary mirror with oppositely directed convex and concave reflective surfaces that reverse both ends of a laser beam propagating from a laser rod disposed between two total internal reflection prisms rigidly positioned with perpendicularly crossed virtual rooflines. The rooflines of the internal reflection prisms are perpendicular to the axis of the laser beam and to the optical axes of the optical resonator components. The unitary mirror with oppositely directed reflective surfaces of opposite sign positioned between opposite ends of the beam enhances the insensitivity of the resonator cavity to misalignment.
In U.S. Pat. No. 3,896,397, de Wit et al. describe an improved acousto-optically Q-switched laser. Corner-cube and Porro prism retro-reflectors are combined and provide improved tolerances for three of the four mounting adjustments. Since the reflecting element near the laser is a corner cube, and since the feedback reflector is a Porro prism, only a single close tolerance adjustment of the reflectors is necessary.
U.S. Pat. No. 3,959,740--Dewhirst discloses a configuration of crystalline quartz wedges utilized as polarizers in combination with an electro-optic switch which constitutes a laser Q-switch. The two wedges are identical and are positioned on either side of the switch. These wedges are oriented such that each compensates for the angular deviation and dispersion of the other. The principal advantage of the disclosed wedge polarizer configuration is that Dewhirst's configuration does not require calcite for the polarizing material, since a large angular separation of the polarizations is not required. The electro-optic switch is oriented with a Porro prism such that in the off condition, both polarizations are misaligned angularly with the Porro prism. The quartz wedges are considerably less expensive and are more easily mounted and aligned in the laser. Quartz is also more durable than calcite and is not as easily damaged by the laser beam.
In U.S. Pat. No. 3,982,203, de Wit discloses an improved, optically pumped, acousto-optically Q-switched laser which produces significantly increased output energy. Optical coatings, with their inherent limitations on the maximum energy of the output pulse, are eliminated. At the reflecting surfaces, Porro prisms replace conventional mirrors. At the nonreflecting or transmitting surfaces, conventional antireflecting coatings are eliminated by placing the respective elements at the Brewster angle for the dominant polarization of the Q-switching material.
In U.S. Pat. No. RE 29,421, Scott discloses a laser system having an electronically selectable gain which employs an acousto-optical beam deflector to variably control the Q of a laser cavity in response to an electronic signal.
None of the preceding inventions completely solves the concomitant problems of polarization conservation, alignment stability, insensitivity to mechanical and thermal changes, and the difficulties of mass producing reflectance outcoupled laser resonators which include a minimum number of reliable components. Such a solution would satisfy a long felt need manifested by the current efforts of the laser and optoelectronics industries, which continue to attempt to develop laser resonators which can cope with the constantly increasing demands for improved performance as described above.